Generally, a displacement measuring device provided with a displacement sensor is used in a variety of appliances such as a magnetically levitated type motor. One example of this type of device is disclosed in Japanese Patent Unexamined Publication No. HEI 6-269144. The displacement sensor which is usually used is referred to as an eddy current type sensor and constituted as shown in FIG. 12. In FIG. 12, an electric current flows through a coil 2 wound around a rod-shaped core body 1 to generate a magnetic flux φr for detection. When an object 3 to be detected such as a metal rotation shaft and the core body 1 are relatively approached to or separated from each other under the magnetic field formed by the magnetic flux φr for detection, the magnitude of eddy current generated in the object 3 varies corresponding to the variation of the distance between the object 3 and the core body 1 and the magnetic reluctance also varies in accordance with its variation. Therefore, a detection output as shown in FIG. 13 is obtained by detecting the variation of inductance for the variation of the magnetic reluctance.
However, in such a conventional type displacement measuring device, the final detection output corresponds to the impedance variation because a DC resistance component is contained in the magnetic reluctance variation including air between the core body 1 and the object 3 to be detected. As a result, in this conventional type displacement sensor, the sensitivity of the detection output is still not sufficient and further, a favorable temperature characteristic is not obtained by the influence of the DC resistance component, the temperature variation of the permeability of the core body or the like. Moreover, since the variation of magnetic reluctance in the air between the core body 1 and the object 3 is proportional to the square of the distance, the linearity of the detection output is not satisfactory. Furthermore, as shown in FIG. 14, when the amount of the variation is enlarged by means of using the variation of three dimensional magnetic flux φr, the size of the entire device is increased. Also, since the impedance is varied by changing the length of a cable connecting with a sensor amplifier, the amplifier is required to be adjusted corresponding to it.
Therefore, it is an object of the present invention to provide a displacement measuring device which is capable of obtaining a stable detection result with simple constitution and appropriate detection sensitivity.